Slip casting, a standard manufacturing procedure used in the manufacture of ceramic products, entails adding a fluid suspension of a ceramic powder, a slip, to a mold constructed of a porous material such as plaster. The fluid migrates into and through the walls of the mold leaving a ceramic powder deposit on the interior wall of the mold. The deposition of the powder on the interior surface of the mold is often facilitated by spinning the mold (centrifugal casting) or by applying pressure to the slip while it is in the mold (pressure casting). Thereafter excess suspension is poured off and the mold removed to produce a green body. Subsequently the green body is dried in a controlled temperature and humidity environment before being subjected to a heat, and usually pressure, treatment such as sintering, hot isostatic pressing, or the like, to substantially fully densify the ceramic powder and increase its strength.
It is known that bodies cast from more highly loaded powder suspensions are more likely to produce net shape castings free from drying cracks. In a net shape casting process, the casting is generally made using only one slip addition to a mold which is essentially the final shape and size of the desired body so that it requires little or no final machining to be ready for use. As a result, net shape castings are characterized by lower drying shrinkage, lessened tendency to warp and crack, and improved dimensional control. Ceramic powder suspensions which contain powder loadings greater than about 70 wt. % are regarded as highly loaded.
Sintering aid oxides such as yttria, magnesia, and the like, are often incorporated with ceramic powders to facilitate the densification process. Generally such sintering aids are added to a ceramic powder during a milling or comminution procedure which reduces the particle size of each and attempts to uniformly mix them. However, it has been found that the desired uniformity of such a mixture is usually less than ideal. As a result, a process was developed to coat such sintering aid metal oxides, or more accurately precursor hydroxides thereto which are readily converted to oxides by the application of heat, onto the surfaces of ceramic partricles. U.S. Pat. Applns. Ser. No. 07/288,208, filed Dec. 22, 1988, now abandoned, and Ser. No. 07/450,200, filed Dec. 13, 1989, the subject matters of which are incorporated herein by reference, disclose inter alia methods for coating the surfaces of a ceramic powder with a solid sintering aid oxide or hydroxide precursor thereto while the powder is dispersed in water. Basically, the method entails controllably precipitating a metal hydroxide or oxide onto the surfaces of an inorganic ceramic particle while substantially avoiding the generation of free sintering aid oxide or hydroxide particles. For example, yttrium hydroxide may be selectively precipitated on the surfaces of silicon nitride by controlling the pH of an aqueous suspension of the silicon nitride to generate an increased reactivity at the surface of the particles as compared to in the bulk. Thus, after an aqueous solution of yttrium nitrate is added to an aqueous slurry of milled and filtered silicon nitride particles and the pH adjusted to about 7, aqueous ammonia solution is slowly added until essentially all of the yttrium is precipitated on the silicon nitride as a coating of yttrium hydroxide. The hydroxide coating can be readily converted to an oxide by heating.
The ceramic bodies which have been cast from such sintering aid coated ceramic particles at concentrations of about 20-40 wt % have a low cast density and have exhibited a high incidence of cracking both in the wet and dry states. Attempts at concentrating the lowly loaded sintering aid coated particles to produce net shape castings have been unsuccessful, apparently due to the the same hydroxide linkages which provide the green strength. As such when the low solids compositions have been concentrated by techniques which have been previously used with other ceramic systems, gels have formed which have prevented increasing the solids content of the suspensions to greater than about 55 wt % prior to its use in pressure casting. Thus, the benefit of the high green strength from using sintering aid coated particles has not been made useful in the production of net shape castings due to the gels which form at high solids loading and which prevent flow of the suspension. It was thought, however, that the crosslinking of the hydroxide gel which occurred could be used to provide adequate wet and dry green strengths to yield a crack-free cast body, provided the casting slip could be concentrated to greater than 65 wt % solids loading, preferably greater than 70 wt %.
Examples of filtration techniques which have been used with suspensions of different ceramic powders include: colloidal filtration of ZrO.sub.2 -toughened alumina from 50 vol % aqueous suspensions; (Konsztowicz et al., Chem. Abs., 113(24):216661r); pressure filtration at 0.3 MPa (Schulle et al., Chem. Abs., 112 (2):10952 g); and a filter to prevent surface defects in slip-cast alumina bodies (Shanefield et al., Chem. Abs. 84(22): 154597w). These previous efforts were not dealing with sintering aid coated ceramic particles and thus did not face the hydroxide gel formation problem which occurs with such particles.
Accordingly, it is an object of this invention to produce aqueous ceramic casting suspensions which are sufficiently highly loaded to yield substantially net shape castings.
More specifically, it is an object of this invention to produce aqueous casting slips containing at least about 65, preferably at least about 70, wt % sintering aid oxide or sintering aid oxide precursor coated ceramic powder which, when pressure cast, yield substantially net shape castings.